1. Field of Applicable Technology
The present invention relates to a method of fingerprint verification which can be utilized, for example to control a keyless entry system, or for controlling entry to a special room such as a computer room.
2. Prior Art Technology
In the prior art, various types of apparatus for fingerprint verification have been envisaged whereby individuals can be respectively identified. Such an apparatus can be used, for example, to restrict entry through a door to one or more authorized individuals, whose fingerprints have been pre-registered and who must present a fingerprint to be recognized by the apparatus whenever entry is required.
An entry control system based on an identification apparatus which employs fingerprints must include a memory device having stored therein one or more registered fingerprints, i.e. respective fingerprint patterns for authorized individuals who have been registered beforehand. When entry is requested to such a system, a registered fingerprint pattern is read out from the memory device and the registered fingerprint pattern is compared with the input fingerprint pattern (i.e. that of the individual who is requesting entry), and a judgement is made as to whether or not the registered fingerprint matches the input fingerprint. With a system having a plurality of registered individuals, each of these generally must first identify himself or herself (e.g. by inputting a specific code number) when requesting entry, whereupon a corresponding registered fingerprint pattern is read out from the memory device to be compared with the input fingerprint.
In the prior art, various methods of implementing such a fingerprint verification apparatus have been envisaged. An example is given in Japanese Patent Laid-Open No. 56-24675. With that apparatus, the termination points and bifurcations of ridges in the fingerprint pattern are detected, as special points within the fingerprint. These points are referred to in the following as minutia. The mutual positional relationships of the minutia of a registered fingerprint and the minutia of an input fingerprint are compared, or the registered fingerprint and the input fingerprint are compared based upon the numbers of ridges which exist between minutia. Verification is thereby executed, indicating whether or not the input fingerprint is the same as the registered fingerprint.
In a method of fingerprint verification described in Japanese Patent Laid-Open No. 58-176781, or the Communications Institute Research Paper PRU87-31, a method of pattern matching for fingerprint verification is proposed which focusses upon portions of the fingerprint pattern in the vicinity of the minutia.
However with such a method of fingerprint verification, since the operation is based on detecting the minutia beforehand and executing verification with these minutia as a reference, problems will arise if the input fingerprint is of poor quality, i.e. if the surface of the input finger is rough, or a portion of the finger surface is of concave shape. In such a case, breaks in the fingerprint ridges appearing in the input fingerprint image will be produced, making it difficult to reliably detect these minutia. Thus, only a low degree of verification reliability is achieved.
Another prior art method of fingerprint verification is disclosed in U.S. Pat. No. 4,581,760. In that method, a pair of small segments of a registered fingerprint are held stored in memory, and are compared with respective portions of an input fingerprint image when verification is required. The registered fingerprint segments and input fingerprint portions are processed in the form of arrays of 2-state pixels (picture elements). Comparison is executed by finding four positions for the registered fingerprint segments relative to the input fingerprint portions, i.e. positions at which there is maximum correlation between the black-state pixels of one registered fingerprint segment and one of the input fingerprint portions, between the white-state pixels of that registered fingerprint segment and input fingerprint portion, between the black-state pixels of the other registered fingerprint segment and the other input fingerprint portion, and between the white-state pixels of that registered fingerprint segment and input fingerprint portion. The criteria for accepting or rejecting an input fingerprint are essentially based upon the mutual relationships of these four positions, however some other additional checking criteria are described by the inventor.
The reason for utilizing a number of different criteria for verifying an input fingerprint with the apparatus of the latter patent, rather than simply making a decision based upon whether a number of mutually coinciding pixels (i.e. a correlation value) is above a certain threshold level, is in order to increase the rate of correct identifications while reducing the rate of incorrect identifications and of incorrect rejections. However it appears that the invention does not in fact provide, in itself, a solution to this problem, since the inventor states that "substantial improvements" in such identification errors can be attained if cascading of two separate fingerprints (i.e. by the individual requesting access) is utilized, and if the individual receives training and practice in placing his or her finger carefully on the platen (of the input photo-rejector).
In this respect, the apparatus of the above patent utilizes comparison of the registered fingerprint segments with only the aforementioned small portions (referred to as "domains") of the input fingerprint, rather than comparing these registered fingerprint segments with the entire input fingerprint. This is done in order to reduce the time required for verification since comparison between each registered fingerprint segment and an input fingerprint domain is executed by sequentially moving a registered fingerprint segment, one pixel at a time, within domain boundaries, and computing the correlation value at each position thus attained. Each position of maximum correlation can then be computed.
Thus, with a prior art method of fingerprint verification in which checking of fingerprint minutia is utilized, the disadvantage arises that satisfactory operation is not attainable if a fingerprint which is to be verified is of poor quality. On the other hand, with a prior art method of fingerprint verification in which each fingerprint is divided into small pixels, and comparison is executed by moving a registered fingerprint relative to an input fingerprint, in steps of pixel units, the disadvantage arises that excessive time is required to execute verification, irrespective of whether the input fingerprint is of poor or of excellent quality, and irrespective of whether the input fingerprint is widely different from the registered fingerprint or contains regions of similarity to the registered fingerprint.
Furthermore with all of these prior art methods of fingerprint verification, achievement of a high overall verification rate (with a low rate of incorrect rejections of input fingerprints) is incompatible with achieving a low rate of incorrect acceptance.